The present invention relates to a demodulator in a receiver, and more particularly to a demodulator of digital modulated signal.
Recently, as satellite broadcasting is being digitized, the demodulator by synchronous detection system of digital modulation method has come to be used in the receiver. Such demodulator is often provided with a function of automatic frequency control (AFC) using a voltage controlled oscillator (VCO).
FIG. 7 is an example of a schematic constitution of a conventional demodulator realizing an AFC circuit using VCO by digital signal processing.
In FIG. 7, a QPSK modulated signal and a local carrier issued from a local oscillator 22 are both put into a mixer 21. In the mixer 21,the QPSK modulated signal is converted in frequency to an intermediate frequency signal (IF signal), and is fed into an orthogonal detector 4 through a band pass filter (BPF) 3. The orthogonal detector 4 converts the IF signal issued from the BPF 3 into orthogonal I, Q base band signals by the transmission output carrier of a VCO 14. By A/D converters (ADC) 5 and 6, analog I, Q base band signals are converted into digital I, Q base band signals. The digitized I, Q base band signals are shaped in waveform by low pass filters (LPF) 7 and 8 of waveform shaping type (digital transversal type) of low pass type, and transmitted to a carrier regenerator 9.
The carrier regenerator 9 detects the phase difference of the phase of the input signal contained in the I, Q base band signals and the reference phase, and issues a reproduction carrier synchronized in phase with the I, Q base band signals from this phase difference signal. A frequency error detector 10 detects a frequency error between the center frequency of QPSK modulated signal and oscillation frequency of the VCO 14, from the phase difference signal of the carrier regenerator 9. This frequency error is smoothed by a frequency controller 11, and converted into an analog signal by a D/A converter (DAC) 13, then transmitted to the VCO 14 as control signal. The operating frequency of the VCO 14 is controlled by the control signal, and the frequency error is compensated.
The frequency error of the center frequency of the QPSK modulated signal and the oscillation frequency of the VCO 14 is always compensated by a feedback loop of AFC composed of the orthogonal detector 4, carrier regenerator 9, frequency error detector 10, frequency controller 11, DAC 13, and VCO 14. Accordingly, the output of the VCO 14 having the same frequency as the QPSK modulation frequency is transmitted to the orthogonal detector 4, and the QPSK modulated signal is synchronously detected in the orthogonal detector 4, and the I, Q base band signals low in error rate are always supplied to the carrier regenerator 9.
In the satellite broadcast, incidentally, dielectric resonators are used in the local oscillator 22 and VCO 14 of the modulator of the BS converter. The dielectric resonators have ambient temperature dependence and time-course changes. Therefore, the frequency of the local oscillator 22 is often detuned from the reference frequency, and its detuning frequency may sometimes reach as much as several mega-Hertz. As for oscillation frequency of the local oscillator 22, in addition to the average frequency detuning from the reference frequency, there is also a frequency drift due to changes of ambient temperature, and this frequency drift cannot be ignored, too.
Further, by using a dielectric resonator in the VCO 14, the voltage-frequency conversion sensitivity characteristic of the VCO 14 has an offset amount, and this offset amount may cause a drift of several mega-Hertz due to time-course changes. Moreover, the inclination of the voltage-frequency conversion sensitivity characteristic may also change in the time course.
In the conventional demodulator described above, if the detuning frequency of the local oscillator 22 surpasses the passing frequency band of the BPF 3, owing to the aforesaid cause, part of the QPSK modulated signal is cut off by the BPF 3, and the error rate characteristic of the demodulator deteriorates significantly. To solve this problem, it has been proposed to detect the detuning frequency of the local oscillator 22 by the frequency error detector 10, and control the local oscillator 22 so that part of the QPSK modulated signal may not be cut off by the BPF 3, but since the detuning frequency value of the local oscillator 22 and the frequency offset value of the voltage-frequency conversion characteristic of the VCO 14 settle in a same AFC feedback loop, the detuning frequency value of the local oscillator 22 and the frequency offset value of the VCO 14 cannot be separated and detected from the frequency information of the AFC.
Besides, as the value of the frequency offset of the voltage-frequency conversion characteristic of the VCO 14 increases, the control frequency band that can be controlled by the AFC becomes narrower, and it may be out of the control frequency band of the AFC depending on the magnitude and direction of the detuning frequency of the local oscillator 22, and control is not converged stably, and demodulation is disabled.
Still more, if the inclination of the voltage-frequency conversion characteristic changes in the time course, the control frequency band controllable by the AFC may be narrower, and the AFC is not converged stably, and demodulation is disabled.
These are the problems of the demodulator by the synchronous detection system of digital modulation system used in the conventional BS converter.
It is hence an object of the invention to solve the above problems, and to present a demodulator in which part of input signal is not cut off by the band pass filter if the detuning frequency of the local oscillator of the demodulator of BS converter surpasses the passing frequency band of the band pass filter, and the frequency offset of the voltage-frequency conversion sensitivity characteristic of the VCO is detected by separating from the detection of the detuning frequency of the local oscillator if the inclination of the voltage-frequency conversion sensitivity characteristic of the VCO changes in the time course, so that the AFC operates stably, thereby preventing deterioration of error rate characteristic or unstable demodulation.
To achieve the object, the invention presents a demodulator comprising a first mixer for converting an input modulated signal into an intermediate frequency signal by frequency conversion, a local oscillator for issuing a local oscillation signal to this mixer, a first band pass filter of band passing type for shaping the waveform of the intermediate frequency signal converted in frequency by the mixer, an orthogonal detector for demodulating the signal shaped in waveform by this filter into orthogonal signals of I, Q channels, a voltage controlled oscillator (VCO) for issuing an oscillation signal to this orthogonal detector, an A/D converter for converting the orthogonal signals of the I, Q channels into digital signals, a second low pass filter of low pass type for shaping the waveform of the output signal from this A/D converter, a carrier regenerator for detecting phase information relating to a carrier from the signal shaped in waveform by this low pass filter for obtaining a generated carrier, a frequency error detector for detecting the frequency error information between the intermediate frequency signal and oscillation frequency of the VCO on the basis of the phase information of this carrier regenerator, a frequency controller for controlling the oscillation frequency of the VCO by the frequency error information detected by this frequency error detector, a signal selector for selecting either the output of this frequency controller or the reference value, a D/A converter for converting the signal from this signal selector into an analog signal, and issuing to the VCO, a frequency divider for dividing the frequency of the output signal of the VCO, a pulse counter for counting signals divided in frequency by this frequency divider for a reference time, and an operator having a function of issuing a selection signal to the signal selector by the output of this pulse counter, and a function of setting the oscillation frequency of the local oscillator by the output of the frequency controller, in which the operator controls the local oscillator so that the frequency of the output signal of the first mixer may exist within the passing frequency band of the first band pass filter, and also settle in the control band which is a frequency band controllable by an AFC feedback loop composed of the orthogonal detector, the carrier regenerator, the frequency error detector, the frequency controller, the signal selector, the D/A converter, and the VCO, and the local oscillator issues a local carrier to the first mixer.
Preferably, the operator selects the reference value in the signal selector, and when the feedback loop of the AFC is opened, the reference value is transmitted to the D/A converter, and is converted into an analog signal. By the analog signal, the VCO is controlled, and the oscillation frequency of the VCO is defined. The output of the VCO is transmitted to the frequency divider. In the frequency divider, the oscillation frequency is divided, and is transmitted to the pulse counter to be counted for a specific time. The operator detects the oscillation frequency of the VCO from the output of the pulse counter. When the reference value is selected, the frequency difference between the design frequency oscillated by the VCO and the frequency measured by the frequency divider and pulse counter is the frequency offset of the VCO, and therefore the operator calculates the frequency band controllable by the AFC by using the calculated frequency offset of the VCO, and calculates the control band in the AFC controllable frequency band and in the passing band frequency band of the band pass filter.
The operator sets the frequency value calculated so that the output signal frequency of the first mixer may settle within the control band, in the local oscillator.
The operator closes the feedback loop of the AFC by using the signal selector.
The input modulated signal is converted into an intermediate frequency signal by the first mixer. The local oscillator is connected to the first mixer, and the oscillation frequency is set by the operator. The intermediate frequency signal is shaped in waveform by the first band pass filter, and is converted into I, Q base band signals by the orthogonal detector. The VCO is connected to the orthogonal detector. The I, Q base band signals converted in frequency by the portion of the frequency of this VCO are converted into a digital value by the A/D converter, and shaped in waveform by the second low pass filter, and put into the carrier regenerator. The carrier regenerator detects the phase information relating to the carrier from the input signal, and obtains a regenerated carrier. The frequency error detector detects the frequency difference information of the intermediate frequency signal and oscillation frequency of the VCO on the basis of the phase information of the carrier regenerator. The output of the frequency error detector is smoothed by the frequency controller, and is converted into an analog signal by the D/A converter. The output of the D/A converter controls the VCO, and corrects the oscillation frequency of the VCO.
The operator reads the frequency converged by the AFC from the output of the frequency controller.
This frequency includes tuning frequency component of the local oscillator of the demodulator and frequency offset component of the VCO, and therefore the operator calculates the detuning frequency component of the local oscillator of the demodulator by using the frequency offset component of the VCO calculated preliminarily.
The operator, using the detuning frequency component of the local oscillator of the demodulator, sets the frequency value calculated so that the output signal frequency of the mixer may settle within the control band, in the local oscillator.
In this constitution, the invention issues the output from the local oscillator of the demodulator to the mixer so that the output signal of the first mixer may settle within the control band, and if the detuning frequency of the local oscillator of the demodulator surpasses the passing frequency band of the first band pass filter, the input modulated signal is not cut off by this first band pass filter to cause deterioration of error rate, and moreover if the controllable frequency band of the AFC is narrowed due to onset of frequency offset of the voltage-frequency conversion sensitivity characteristic of the VCO, not running away from the controllable frequency band, the feedback loop of the AFC is always converged stably.